Galaxies in the Universe
Galaxies: From Here to the Horizon
Thus the explorations of space end on a
note of uncertainty… Eventually, we reach
the dim boundary – the utmost limits of
our telescopes. There we measure
shadows, and we search among ghostly
errors of measurement for landmarks that
are scarcely more substantial.
Edwin Hubble (1889 – 1953)
American astronomer
WHAT DO YOU THINK?
1.
2.
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4.
How far away are galaxies?
Do all galaxies have spiral arms?
Are galaxies isolated objects?
Is the universe contracting, unchanging in
size, or expanding?
In this chapter you will discover…
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galaxies are categorized by their shapes
processes that produce galaxies of different
shapes
galaxies are found in clusters that contain huge
amounts of dark matter
why clusters of galaxies form in superclusters
how some galaxies merge and others devour
their neighbors
the universe is changing size
Final Exam Essay Questions
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Essay Questions on the final relating to this
chapter include:
 What kinds of galaxies are there in the
universe? How do we measure distances to
galaxies?
 What is Hubble's law? What does it tell us
about the evolution of the universe?
 What is the missing mass problem? What
does it mean about the evolution of the
universe?
Process of Science
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Observe galaxies at all distances
Research properties, similarities, patterns
Hypotheses:
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Why are they different?
When did they form?
How do they evolve?
Experiments?
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Look over distance (and time!)
Put into a timeline
Spirals
Variations?
Spirals
Variations?
Size of nucleus, amount of winding,
“fluffiness”
Spirals
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have a disk component and bulge & halo
disk contains an ISM of gas & dust
relative sizes of bulge/disk & amount of gas/dust vary
appear white because they contain both blue & red stars
Spirals “Edge-on”
Galaxies don’t seem to “wind up”
over time!
Spiral Arms as Density “Traffic Jams”
Barred Spirals
Elliptical Galaxies
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only have a spheroidal component; no disk
very little gas/dust, little active star formation
appear red because they contain mostly red stars
Elliptical Galaxy Variations
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Size of central nucleus
Degree of “flatness” or “ellipticity
Irregular Galaxy Variations
•“none of the above”; neither spiral nor elliptical
•appear white & dusty: lots of gas & dust
•have more in common w/ disk component of
spirals
•distant galaxies more likely to be irregular
•So…more common when the Universe was
Edwin Hubble
(1889-1953)
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Discovered Cepheid variables
in Andromeda galaxy.
Calculated distance to
Andromeda galaxy.
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2.2 million light years
not in the Milky Way
Developed a classification
scheme for galaxies.
Hubble’s Classification System
Milton Humason
(1891-1972)
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Mule-team driver who
dropped out of 8th grade.
Took spectra & measured
the redshifts of galaxies.
Worked w/ Hubble, who
measured the distances to
those same galaxies.
They plotted distance vs.
velocity
The Cosmic Distance Ladder
The Cosmic Distance Ladder
Parallax to
Nearby Stars
~500 light years
The Cosmic Distance Ladder
Spectroscopic
Parallax to
Stars in Galaxy
~30,000 light years
The Cosmic Distance Ladder
Calibrated
Variable Stars
in Galaxy
~50,000 light years
The Cosmic Distance Ladder
Calibrated Variable
Stars, Supernovae in
“Nearby” Galaxies
~10 milion light years
The Cosmic Distance Ladder
Calibrated Variable Stars,
Supernovae, Most massive
galaxies in “Distant”
Clusters
~500 million to 5 billion
light years
The Cosmic Distance Ladder
Hubble’s Law
~10 billion light years
Cosmic Distance Ladder!
Hubble’s Law
Distance of
Galaxies is
indicated by
REDSHIFT of star
light.
Hubble’s Law: Velocity of Distant Galaxies
Increases with Distance
Farther Galaxies move
away from us faster
Closer Galaxies move
away from us
Hubble’s Constant indicates AGE of Universe!
Hubble’s Law:
Velocity = H0 x Distance
Hubble’s Constant Tells
Age:
H0 = Velocity/Distance =
1 / Time
Galactic “Cannibalism”
More Evidence of Dark Matter
Gravitational Lenses Suggest Dark Matter
Summary of Key Ideas
Types of Galaxies
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The Hubble classification system groups galaxies into
four major types: spiral, barred spiral, elliptical, and
irregular.
The arms of spiral and barred spiral galaxies are sites of
active star formation.
According to the theory of self-propagating star
formation, spiral arms of flocculent galaxies are caused
by the births and deaths of stars over extended regions
of a galaxy. Differential rotation of a galaxy stretches the
star-forming regions into elongated arches of stars and
nebulae that we see as spiral arms.
Types of Galaxies
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According to the spiral density wave theory, spiral arms
of grand-design galaxies are caused by density waves.
The gravitational field of a spiral density wave
compresses the interstellar clouds that pass through it,
thereby triggering the formation of stars, including OB
associations, which highlight the arms.
Elliptical galaxies contain much less interstellar gas and
dust than do spiral galaxies; little star formation occurs in
elliptical galaxies.
Clusters and Superclusters
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Galaxies group into clusters rather than being randomly
scattered through the universe.
A rich cluster contains at least a thousand galaxies; a
poor cluster may contain only a few dozen up to a
thousand galaxies. A regular cluster has a nearly
spherical shape with a central concentration of galaxies;
in an irregular cluster, the distribution of galaxies is
asymmetrical.
Our Galaxy is a member of a poor, irregular cluster,
called the Local Group.
Rich, regular clusters contain mostly elliptical and
lenticular galaxies; irregular clusters contain more spiral
and irregular galaxies. Giant elliptical galaxies are often
found near the centers of rich clusters.
Clusters and Superclusters
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No cluster of galaxies has an observable mass large
enough to account for the observed motions of its
galaxies; a large amount of unobserved mass must be
present between the galaxies.
Hot intergalactic gases emit X rays in rich clusters.
When two galaxies collide, their stars initially pass each
other, but their interstellar gas and dust collide violently,
either stripping the gas and dust from the galaxies or
triggering prolific star formation. The gravitational effects
of a galactic collision can cast stars out of their galaxies
into intergalactic space.
Galactic mergers occur; a large galaxy in a rich cluster
may grow steadily through galactic cannibalism,
sometimes producing a giant elliptical galaxy.
Superclusters in Motion
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A simple linear relationship exists between the distance
from Earth to galaxies in other superclusters and the
redshifts of those galaxies (a measure of the speed at
which they are receding from us). This relationship is
the Hubble law: recessional velocity = Ho x distance,
where Ho is the Hubble constant.
Astronomers use standard candles—Cepheid variables,
the brightest supergiants, globular clusters, H II regions,
supernovae in a galaxy, and the Tully-Fisher relation—
to calculate intergalactic distances. Because of
difficulties in measuring the distances to remote
galaxies, the value of the Hubble constant, Ho, is not
known with complete certainty.
Key Terms
barred spiral galaxy
cluster (of galaxies)
elliptical galaxy
galactic merger
gravitational lensing
Hubble classification
Hubble constant
Hubble flow
Hubble law
intergalactic gas
irregular cluster (of galaxies)
irregular galaxy
lenticular galaxy
Local Group
poor cluster (of galaxies)
regular cluster (of galaxies)
rich cluster (of galaxies)
spiral density wave
spiral galaxy
standard candle
starburst galaxy
supercluster (of galaxies)
trailing-arm spiral galaxy
Tully-Fisher relation
WHAT DID YOU THINK?
Are most of the stars in spiral galaxies
located in their spiral arms?
 No. The spiral arms contain only 5% more
stars than the regions between the arms.
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WHAT DID YOU THINK?
Do all galaxies have spiral arms?
 No. Galaxies may be either spiral, barred
spiral, elliptical, or irregular. Only spirals
and barred spirals have arms.
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WHAT DID YOU THINK?
Are galaxies isolated objects?
 No. Galaxies are grouped in clusters, and
clusters are grouped in superclusters.
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WHAT DID YOU THINK?
Is the universe contracting, unchanging in
size, or expanding?
 The universe is expanding.
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